Canada last week confirmed the sixth case in an animal that was at least 15 years old, and born before Canada implemented restrictions on potentially dangerous feed in 1997.

Mad cow disease is believed to spread through feed, when cows eat the contaminated tissue of other cattle. Humans can get a related disease, variant Creutzfeldt-Jakob Disease, by eating meat contaminated with mad cow. There have been more than 150 human deaths worldwide linked to the variant.

Two of the six confirmed mad cow cases in Canada have involved animals that were infected after 1997, when a ban was instituted on the use of cattle parts in feed for cattle, or other ruminants such as sheep and goats.

Shipments of live cattle to the United States were halted in 2003 after the first reported mad cow case in Canada. Trade in young animals resumed last year, but there has been no word on when the border may be reopened to older animals.